EE 122: IP Addressing
Ion Stoica
TAs: Junda Liu, DK Moon, David Zats
http://inst.eecs.berkeley.edu/~ee122/
(Materials with thanks to Vern Paxson, Jennifer Rexford,
and colleagues at UC Berkeley)
1
Goals of Today’s Lecture

IP addresses






Dotted-quad notation
IP prefixes for aggregation
Classful addresses
Classless InterDomain Routing (CIDR)
Special-purpose address blocks
Address allocation


Hierarchy by which address blocks are given out
Finding information about an allocation
2
Designing IP’s Addresses

Question #1: what should an address be
associated with?



E.g., a telephone number is not associated with a
person but with a handset
Question #2: what structure should addresses
have? What are the implications of different
types of structure?
Question #3: who determines the particular
addresses used in the global Internet? What are
the implications of how this is done?
3
IP Addresses (IPv4)



A unique 32-bit number
Identifies an interface (on a host, on a router, …)
Represented in dotted-quad notation. E.g,
12.34.158.5:
12
34
158
5
00001100 00100010 10011110 00000101
4
Hierarchical Addressing in U.S.
Mail

Addressing in the U.S. mail






Zip code: 15232
Street: Forbes Avenue
Building on street: 5000
Apartment: 61B
Name of occupant: Ion Stoica
???
Forwarding the U.S. mail




Deliver letter to the post office in the zip code
Assign letter to mailman covering the street
Drop letter into mailbox for the building/room
Give letter to the appropriate person
5
Hierarchical Addressing: IP
Prefixes


Divided into network (left) & host portions (right)
12.34.158.0/24 is a 24-bit prefix with 29 addresses

Terminology: “Slash 24”
12
34
158
5
00001100 00100010 10011110 00000101
Network (24 bits)
Host (8 bits)
6
IP Address and a 24-bit Subnet Mask
Address
12
34
158
5
00001100 00100010 10011110 00000101
11111111 11111111 11111111 00000000
Mask
255
255
255
0
7
Addressing Hosts in the Internet

The Internet is an “inter-network”


Used to connect networks together, not hosts
Needs a way to address a network (i.e., group of
hosts)
host
host ...
host
host
host ...
host
LAN 2
LAN 1
router
WAN
LAN = Local Area Network
WAN = Wide Area Network
router
WAN
router
8
Routers

Router consists of




Set of input interfaces where packets arrive
Set of output interfaces from which packets depart
Some form of interconnect connecting inputs to outputs
Router implements


Forward packet to corresponding output interface
Manage bandwidth and buffer space resources
host
host ...
host
host
host ...
host
LAN 2
LAN 1
router
WAN
router
WAN
router
Router
9
Forwarding Table

Store a mapping between IP addresses and output
interfaces

Forward an incoming packet based on its destination address
1.2.3.5
1.2.3.6
1.2.3.4
1
3
2
… …
1.2.3.4
1
1.2.3.5
2
10
Scalability Challenge

Suppose hosts had arbitrary addresses


Then every router would need a lot of information
…to know how to direct packets toward the host
1.2.3.4
5.6.7.8
host
host ...
2.4.6.8
host
1.2.3.5
5.6.7.9
host
host ...
2.4.6.9
host
LAN 2
LAN 1
router
WAN
router
WAN
router
1.2.3.4
1.2.3.5
forwarding table
11
Scalability Improved

Number related hosts from a common subnet


1.2.3.0/24 on the left LAN
5.6.7.0/24 on the right LAN
1.2.3.4
1.2.3.7 1.2.3.156
host ...
host
5.6.7.8 5.6.7.9 5.6.7.212
host
host
host ...
host
LAN 2
LAN 1
router
WAN
router
WAN
router
1.2.3.0/24
5.6.7.0/24
forwarding table
12
Easy to Add New Hosts

No need to update the routers


E.g., adding a new host 5.6.7.213 on the right
Doesn’t require adding a new forwarding entry
1.2.3.4
1.2.3.7 1.2.3.156
host ...
host
5.6.7.8 5.6.7.9 5.6.7.212
host
host
host ...
host
LAN 2
LAN 1
router
WAN
router
WAN
router
host
5.6.7.213
1.2.3.0/24
5.6.7.0/24
forwarding table
13
Classful Addressing

Class A: if first byte in [0..127], assume /8 (top bit = 0)
0******* ******** ******** ********


Class B: first byte in [128..191]  assume /16 (top bits = 10)


Very large blocks (e.g., MIT has 18.0.0.0/8)
10****** ******** ******** ********
Large blocks (e.g,. UCB has* 128.32.0.0/16)
Class C: [192..223]  assume /24 (top bits = 110)
110***** ******** ******** ********


Small blocks (e.g., ICIR has 192.150.187.0/24)
The “swamp” (many European networks, due to history)
14
Classful Addressing (cont’d)

Class D: [224..239] (top bits 1110)
1110**** ******** ******** ********


Class E: [240..255] (top bits 11110)


Multicast groups
11110*** ******** ******** ********
Reserved for future use
What problems can classful addressing lead to?


Only comes in 3 sizes
Routers can end up knowing about a lot of class C’s
15
Classless Inter-Domain Routing (CIDR)
Use arbitrary length prefixes
Use two 32-bit numbers to represent a network.
Network number = IP address + Mask
IP Address : 12.4.0.0
Address
Mask
IP Mask: 255.254.0.0
00001100 00000100 00000000 00000000
11111111 11111110 00000000 00000000
Network Prefix
for hosts
Written as 12.4.0.0/15 or 12.4/15
16
CIDR: Hierarchal Address Allocation

Prefixes are key to Internet scalability


Addresses allocated in contiguous chunks (prefixes)
Routing protocols and packet forwarding based on prefixes
12.0.0.0/15
12.2.0.0/16
12.3.0.0/16
12.0.0.0/8
:
:
12.253.0.0/16
:
12.3.0.0/22
12.3.4.0/24
:
:
:
:
:
12.3.254.0/23
12.253.0.0/19
12.253.32.0/19
12.253.64.0/19
12.253.64.108/30
12.253.96.0/18
12.253.128.0/17
17
Scalability: Address Aggregation
Provider is given 201.10.0.0/21
(201.10.0.x .. 201.10.7.x)
Provider
201.10.0.0/22
201.10.4.0/24
201.10.5.0/24
201.10.6.0/23
Routers in the rest of the Internet just need to know
how to reach 201.10.0.0/21. The provider can direct the
IP packets to the appropriate customer.
18
But, Aggregation Not Always
Possible
201.10.0.0/21
Provider 1
Provider 2
201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23
Multi-homed customer with 201.10.6.0/23 has two
providers. Other parts of the Internet need to know how
to reach these destinations through both providers.
19
 /23 route must be globally visible
Growth in Routed Prefixes (1989-2005)
Dot-com implosion;
Internet bubble bursts
Advent of CIDR
allows aggregation:
linear growth
Initial growth
super-linear; no
aggregation
Back in
business
Internet boom:
multihoming drives
superlinear growth
20
Special-Purpose Address Blocks

Private addresses

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Link-local

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By agreement, not forwarded by any router
Used for single-link communication only
Intent: autoconfiguration (especially when DHCP fails)
Block: 169.254.0.0/16
Loopback




By agreement, not routed in the public Internet
For networks not meant for general Internet connectivity
Blocks: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
Address blocks that refer to the local machine
Block: 127.0.0.0/8
Usually only 127.0.0.1/32 is used
Limited broadcast


Sent to every host attached to the local network
Block: 255.255.255.255/32
21
Scalability Through Non-Uniform
Hierarchy
Summary:
 Hierarchical addressing




Critical for scalable system
Don’t require everyone to know everyone else
Reduces amount of updating when something
changes
Non-uniform hierarchy



Useful for heterogeneous networks of different sizes
Initial class-based addressing was far too coarse
Classless InterDomain Routing (CIDR) gains much
more flexibility
22
5 Minute Break
Questions Before We Proceed?
23
Address Allocation
24
Obtaining a Block of Addresses

Separation of control



Prefix: assigned to an institution
Addresses: assigned by the institution to their nodes
Who assigns prefixes?

Internet Corporation for Assigned Names and Numbers

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
Regional Internet Registries (RIRs)


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Allocates large address blocks to Regional Internet Registries
ICANN is politically charged
E.g., ARIN (American Registry for Internet Numbers)
Allocates address blocks within their regions
Allocated to Internet Service Providers and large institutions ($$)
Internet Service Providers (ISPs)

Allocate address blocks to their customers (could be recursive)

Often w/o charge
25
Figuring Out Who Owns an Address

Address registries



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Public record of address allocations
Internet Service Providers (ISPs) should update when
giving addresses to customers
However, records are notoriously out-of-date
Ways to query



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UNIX: “whois –h whois.arin.net 169.229.60.27”
http://www.arin.net/whois/
http://www.geektools.com/whois.php
…
26
Example Output for 169.229.60.27
University of California, Office of the President UCNETBLK (NET-169-229-0-0-1)
169.229.0.0 - 169.233.255.255
University of California at Berkeley ISTDATA
(NET-169-229-0-0-2)
169.229.0.0 - 169.229.255.255
•
ISTDATA – Information Services and Technology (IST) Data Communication and Network Services
27
Are 32-bit Addresses Enough?

Not all that many unique addresses




And, many devices need IP addresses


Computers, PDAs, routers, tanks, toasters, …
Long-term solution (perhaps): larger address space


232 = 4,294,967,296 (just over four billion)
Plus, some (many) reserved for special purposes
And, addresses are allocated in larger blocks
IPv6 has 128-bit addresses (2128 = 3.403 × 1038)
Short-term solutions: limping along with IPv4



Private addresses
Network address translation (NAT)
Dynamically-assigned addresses (DHCP)
28
Network Address Translation (NAT)

Before NAT…

Every machine connected to the Internet had a
unique IP address
Server
80 1001 5.6.7.8 1.2.3.4 Internet
5.6.7.8
src port
dest addr src addr
dst port
LAN
5.6.7.8 1.2.3.4 80 1001
1.2.3.4
1.2.3.5
Clients
29
Network Address Translation (cont’d)

Independently assign addresses to machines behind
same NAT


Usually in address block 192.168.0.0/16
Use port numbers to multiplex demultiplex internal
addresses
Server
NAT 5.6.7.8 192.2.3.4 80 1001
80 2000 5.6.7.8 1.2.3.4
192.2.3.4
Internet
5.6.7.8
80 1001
1.2.3.4
5.6.7.8
80 2000
192.2.3.4
5.6.7.8
1.2.3.4
192.2.3.4:1001
1.2.3.4:2000
192.2.3.5
Clients
30
Network Address Translation (cont’d)

Independently assign addresses to machines behind
same NAT


Usually in address block 192.168.0.0/16
Use port numbers to multiplex demultiplex internal
addresses
Server
NAT
80 2001 5.6.7.8 1.2.3.4
192.2.3.4
Internet
5.6.7.8 1.2.3.4
1.2.3.4 80 2001
5.6.7.8
80 1001 5.6.7.8 192.2.3.5
192.2.3.4:1001
5.6.7.8 192.2.3.5 80 1001
192.2.3.5
1.2.3.4:2000
192.2.3.5:1001
1.2.3.4:2001
Clients
31
Hard Policy Questions

How much address space per geographic region?




Address space portability?




Equal amount per country?
Proportional to the population?
What about addresses already allocated?
Keep your address block when you change providers?
Pro: avoid having to renumber your equipment
Con: reduces the effectiveness of address aggregation
Keeping the address registries up to date?



What about mergers and acquisitions?
Delegation of address blocks to customers?
As a result, the registries are often out of date
32
Summary of IP Addressing



32-bit numbers identify interfaces
Allocated in prefixes
Non-uniform hierarchy for scalability and flexibility



A number of special-purpose blocks reserved
Address allocation:


Routing is based on CIDR
ICANN  RIR  ISP  customer network  host
Issues to be covered later


How hosts get their addresses (DHCP)
How to map from an IP address to a link address (ARP)
33
Next Lecture


IP Forwarding; Transport protocols
Read K&R: 3-3.4 (pp 195-240)
34